Receiver with automatic gain control that operates with multiple protocols and method thereof

ABSTRACT

An automatic gain control (AGC) method and circuit ( 10 ) within a receiver uses a digital state machine ( 26 ) to implement the AGC function. independent from interaction with a host processor ( 36 ) and for multiple modulation protocols without duplicating circuitry. Modulation protocol and parameters for any of various gain responses are stored in a register ( 29 ). Multiple states, each corresponding to a predetermined range of RF input signal strength, are stored in the register. Each state contains parameters that determine a gain control signal for controlling a variable gain amplifier ( 16 ). The states are independent and may be selectively disabled to create asymmetric responses. Within any state, an adaptable number of iterations may be set to implement a different update rate or step size after a predetermined number of closed loop gain change iterations has not resulted in a transition to a state that represents a desired output gain.

FIELD OF THE INVENTION

This invention relates generally to communication receivers, and morespecifically, to automatic gain control (AGC) in a receiver.

BACKGROUND OF THE INVENTION

Radio Frequency (RF) receivers typically incorporate Automatic GainControl (AGC) circuitry to provide proper conditioning of the receivedRF input power such that the received signal is kept within the usabledynamic range of the receiver. The information embedded in the receivedsignal is transported using one of many different modulation schemeswherein the information may be contained in the frequency or phase ofthe received signal (e.g. FM, PM, FSK, PSK, etc.), in the amplitude ofthe received signal (eg. AM), or in both the amplitude and phase of thereceived signal (eg. QAM). Receiver AGC requirements are driven byseveral modulation and protocol parameters including, but not limitedto, peak-to-average power ratio of the modulation, demodulator dynamicrange limitations, analog gain/filtering compression (linearity)responses specific to the particular receiver, protocol driven slottedtiming structures, and synchronous versus asynchronous systemrequirements. Examples of current protocol structures that incorporatedistinct AGC system requirements are IS95 (commonly known as CDMA), GSM,iDEN, ETSI EN 300392 known as TErrestrial Trunked Radio (TETRA) and APCO25. Another standard is the TIA 902 Scalable Advanced Modulation (SAM)standard that is a 700 MHz public domain standard that may be used forpublic safety applications. In present receivers, AGC system design isgenerally tailored to specific protocols and modulation strategies.

As has been previously noted, the modulation's peak-to-average ratiogreatly influences the selection of the AGC threshold. If theinformation within the received signal is contained only in the phasecomponent (e.g. FM or PM), the AGC strategy is greatly simplified sincethe modulated information is not lost even when the receiver isoperating in compression. However, as increasing portions of theinformation are contained in the amplitude component of the receivedsignal, as indicated by increasing peak-to-average ratios of thereceived signal, the receiver linearity requirements greatly increasethus necessitating increased AGC complexity. With increasingpeak-to-average levels, the AGC thresholds are selected so as to keepthe receiver operating completely out of compression. Compression is anoperating state typically encountered in strong RF input powerconditions where an amplifier stage looses its small signal gaincharacteristic. Compression results in the loss of all or a portion ofthe amplitude component of the modulated information. Therefore, as thepeak-to-average ratio of the modulation increases, it becomesincreasingly critical for the AGC to engage sufficient attenuation toprevent the amplifier from operating in a state of compression.

Another important aspect of the AGC system design involves trade-offsbetween attack time and tracking characteristics as determined by themodulation scheme of the received signal and by protocol specific timingrequirements. The AGC tracking rate must be set to avoid distorting thereceived signal, particularly in the form of undesired amplitude rippleof the received RF carrier induced by the closed loop AGC continuouslytracking the signal level. This distortion is particularly detrimentalto a received signal containing significant amplitude component withinits modulation. To reduce the AM distortion effect, the AGC trackingrate (which is inversely proportional to the closed loop bandwidth ofthe AGC) must be slowed down such that the AGC cannot respond quickly toamplitude variations in the RF carrier induced by the modulation scheme.Slow AGC tracking rates are desirable for highly linear modulationstrategies that incorporate a large amplitude component within the RFcarrier, since fast AGC tracking of linear modulation strategies willresult in the AGC tracking out the desired amplitude portion of themodulation. However, in a simplified closed loop control system, slowingdown the AGC tracking rate has the undesired effect of increasing theAGC attack time. The AGC attack time is the duration required for theAGC to engage the required attenuation to achieve proper demodulationonce the receiver has encountered an arbitrary change in RF input powerlevel. Most modern protocol structures require fast AGC attack times.For a basic closed loop feedback system, the fast AGC attack timerequirement is in direct conflict with the requirement to minimize AGCinduced amplitude distortion of the desired signal. Therefore, thereexists a paradox in receiver systems, where particular protocols mayrequire fast AGC attack times necessitating high AGC tracking rates,while the highly linear modulation strategy incorporated into the sameprotocol may require slow AGC tracking rates that would degrade attacktimes. Previous receivers have attempted to resolve this paradox byfocusing on specific protocols and modulation strategies without regardto readily adapting the system to accommodate multiple protocols andmodulation types. It is advantageous for receivers to be readilyadaptable to accommodate the numerous modulation strategies and receiverprotocols that exist today.

AGC strategies are further complicated by protocol requirementsnecessitating AGC response to both synchronous and asynchronous signals.Some of these modulation strategies have specific timing requirementswhere the desired data is contained within specific slots of time for agiven duration. Such strategies are synchronous, and are known as TimeDivision Multiple Access (TDMA) protocols. The same protocol can defineanother mode which is asynchronous, allowing direct radio-to-radiooperation. For example, the TETRA protocol defines a TDMA Trunked Modeof Operation (TMO) and a radio-to-radio mode of operation known asDirect Mode Operation (DMO). In the DMO mode, the receiver is requiredto receive a discontinuous TDMA signal from another radio. Thus, thereceiver AGC settling time should be extremely fast. For multi-nationalwireless communication companies, it is a competitive advantage todefine common platforms that are able to meet these diverse protocoltiming and modulation linearity requirements. Therefore, the AGCoperation must vary significantly for each of these standards, and thereceiver hardware must be adapted for all targeted standards, protocolsand modulation techniques. Existing systems that are targeted tomultiple standards incur significantly increased costs and/orperformance degradation due to increased hardware complexity and/orincreased system resource demands (i.e. increased host processingresulting in increased power consumption and increased latency inservicing user specific applications).

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention is illustrated by way of example and is notlimited by the accompanying figures, in which like references indicatesimilar elements.

FIG. 1 illustrates in block diagram form a portion of a receiver havingan automatic gain control circuit in accordance with the presentinvention;

FIG. 2 illustrates in tabular form various state conditions associatedwith a digital controller of the automatic gain control circuit of FIG.1;

FIG. 3 illustrates a simplified flow chart of a method of automatic gaincontrol in accordance with the present invention;

FIG. 4 illustrates a more detailed flow chart of the method of automaticgain control in accordance with the present invention;

FIG. 5 illustrates in graphical form the relationship of multiplethresholds relative to input signal level and associated changes of AGCstate parameters that determine attenuation step sizes;

FIG. 6 illustrates in graphical form the relationship of multiplethresholds relative to input signal level and associated changes of AGCstate parameters that determine attenuation update rates;

FIG. 7 illustrates in flow chart form a simplified method of automaticgain control where mono-directional gain adjustment is implemented; and

FIG. 8 illustrates in flow chart form a method to further enhance AGCperformance for a specific time limit within contiguous thresholdboundaries in accordance with the present invention.

Skilled artisans appreciate that elements in the figures are illustratedfor simplicity and clarity and have not necessarily been drawn to scale.For example, the dimensions of some of the elements in the figures maybe exaggerated relative to other elements to help improve theunderstanding of the embodiments of the present invention.

DETAILED DESCRIPTION

Illustrated in FIG. 1 is a receiver 10 with automatic gain control foroperating with multiple modulation protocols and which interfaces with ahost processor 36 through a serial peripheral interface (SPI) 34. Forconvenience of illustration and emphasis on automatic gain control, allthe circuitry associated with a receiver is not illustrated. Thereceiver 10 has an antenna 12 connected to an input of a step attenuator14. Step attenuator 14 has an output connected to an input of a variablegain amplifier 16. An output of the variable gain amplifier 16 isconnected to a first input of a mixer 18. A second input of mixer 18 isconnected to a Local Oscillator (LO) signal. An output of mixer 18 isconnected to an input of a filter 20. An output of the filter 20 isconnected to an input of an on-channel signal detector 22 and providesan output signal to be connected to a demodulator (not shown). An outputof the variable gain amplifier 16 is connected to an input of anoff-channel signal detector 24. An output of on-channel signal detector22 is connected to a first input of an automatic gain control statemachine controller 28. An output of off-channel signal detector 24 isconnected to a second input of the automatic gain control state machinecontroller 28. Automatic gain control state machine controller 28 ismade up of conventional logic circuitry for implementing the variouscircuit states and functions described herein. An output of theautomatic gain control state machine controller 28 is connected to aninput of a digital-to-analog converter 30, also known as a DAC. Anoutput of the digital-to-analog converter 30 is connected to a controlinput of the variable gain amplifier 16. A second output of theautomatic gain control state machine controller 28 is connected to acontrol input of the step attenuator 14. A clock divider 32 has an inputfor receiving an input clock of predetermined frequency. An output ofclock divider 32 is connected to a clock input of the automatic gaincontrol state machine controller 28. A storage device implemented as aconventional register is identified as the modulation and protocol andparameter registers 29. The modulation and protocol and parameterregisters 29 have outputs respectively connected to a third input of theautomatic gain control state machine controller 28 and to a firstcontrol input of the clock divider 32. The serial peripheral interface34 has an input connected to an output of the host processor 36 and anoutput connected to a second control input of the clock divider 32, toan input of the modulation protocol and parameter registers 29 and to afourth input of the automatic gain control state machine controller 28.In combination, the clock divider 32, the automatic gain control statemachine controller 28, the digital-to-analog converter 30 and themodulation protocol and parameter registers 29 form a state machine 26.

In operation, a radio frequency (RF) signal is received and attenuatedsufficiently by either the step attenuator 14 under control of theautomatic gain control state machine controller 28 or the variable gainamplifier 16 depending upon the input power level of the receivedsignal. It should be understood that step attenuator 14 is an optionalcircuit component and step in the method of gain control disclosedherein. The variable gain amplifier 16 is a low noise amplifier thatfunctions to amplify the received RF signal information content withoutproportionately amplifying the noise content therein. Mixer 18 functionsto translate the RF input signal to an intermediate frequency (IF)signal which is then filtered by filter 20 that may be implemented as abandpass or a low pass filter. This frequency translation requires theapplication of local oscillator signal to mixer 18 with the LocalOscillator frequency having the mathematical relationship to the RFinput carrier that produces the desired intermediate frequency. Thefilter 20 provides narrow band selectivity so as to attenuate signalsthat are not the desired RF input signal. The level of attenuation isdependent on the filter characteristics and the magnitude of the offsetbetween the desired RF input signal and any other signal that may beattenuated by the filter 20. The output of filter 20 is connected to ademodulator (not shown) and to the on-channel signal detector 22. Theon-channel signal detector 22 produces a digital word that indicates thelevel of the signal swing at the output of filter 20. The on-channelsignal detector 22 may be implemented by a variety of differentconventional circuits. By way of illustration only, on-channel signaldetector 22 may be implemented with a signal detector (not shown)connected in series with an integrator (not shown) that is connected toan analog-to-digital (A/D) converter (not shown) for providing a digitaloutput to state machine 26. The signal detector may be implemented withany of conventional detectors such as a sum of squares detector, a fullwave rectifier detector, a root sum squares detector or a peak detector.The signal detector provides a signal that represents the signalstrength, which is then integrated prior to being converted to a digitalrepresentation. This digital word at the output of on-channel signaldetector 22 is processed by the automatic gain control state machinecontroller 28 to determine the AGC attenuation response. In addition,the output of variable gain amplifier 16 is connected to off-channelsignal detector 24. The off-channel signal detector 24 produces adigital word indicating the signal strength of the unfiltered signallevel at the output of the variable gain amplifier 16. This may includeinput power levels of signals that are not the desired RF carrier, andmay exhibit significantly increased power levels. By way of illustrationonly, off-channel signal detector 24 may be implemented with an RFdetector (not shown) connected in series with an integrator (not shown)that is connected to an analog-to-digital (A/D) converter (not shown)for providing a second digital output to state machine 26 thatrepresents a D.C. signal that is proportional to RF signal output ofvariable gain amplifier 16. The RF detector (not shown) provides anoutput from the RF signal that is similar to a full wave rectifiedsignal meaning that the RF detector output is a D.C. signal having anA.C. component. The integrator (not shown) functions as a low passfilter. Therefore, the off-channel signal detector 24 provides adigitized quantity of a D.C. signal that is proportional to the RFsignal output of variable gain amplifier 16. The output of theoff-channel signal detector 24 is also processed by the automatic gaincontrol state machine controller 28 in conjunction with the output ofthe on-channel signal detector 22. Other structural implementations ofthe on-channel signal detector 22 and off-channel signal detector 24 maybe used. For example, the internal structures of the on-channel signaldetector 22 and the off-channel signal detectors 24 may be similar andmay include, but is not limited to, other circuitry for rectifying theRF signals to produce a proportional D.C. voltage and formatting theD.C. voltage into a digital word that indicates the magnitude of theD.C. voltage. State machine 26 functions as a digital controller tocontrol the receiver gain by controlling variable gain amplifier 16 andimplements this function without requiring monitoring by the hostprocessor 36, thereby allowing the host processor 36 to perform otherfunctions while the AGC function is occurring. The automatic gaincontrol state machine controller 28 processes the input digital wordsfrom the on-channel signal detector 22 and the off-channel signaldetector 24 based on the control parameters stored in modulationprotocol and parameter registers 29 or programmed by the serialperipheral interface 34 at a speed controlled by the frequency of theclock signal provided-by clock divider 32. The conditions indicated bythe off-channel signal detector 24 and the on-channel signal detector 22in conjunction with the parameters in the modulation protocol andparameter registers 29 determine digital-to-analog converter 30 outputstimulus that controls the variable gain amplifier 16 attenuation andthe step attenuator 14 state (i.e. whether step attenuator 14 is engagedor disengaged).

The parameters in the modulation protocol and parameter registers 29 areprovided from the host processor 36 prior to the receive operationdiscussed herein. The host processor 36 uses the serial peripheralinterface 34 to set control information corresponding to a desiredattenuation control response. The host processor 36 performs thisfunction once during an initialization and is then free from servicingon-going internal maintenance functions and user-specific applicationrequirements. It should be noted that this feature is a significantsavings in system resources as the host processor 36 is not required toservice the AGC function.

Illustrated in FIG. 2 is a table that represents a multiple statepartitioning of the overall AGC response embedded into the automaticgain control state machine controller 28 of FIG. 1. FIG. 2 illustratesfive states in detail with up to N states possible, where N is aninteger. Each state has an independent AGC action responding to specificinput signal levels as determined by the digital word indicated from theon-channel signal detector 22 and the off-channel signal detector 24. Itis further noted that the specific AGC action within each state isdetermined by the automatic gain control state machine controller 28 bysetting the update rate and the AGC step size at the output ofdigital-to-analog converter 30. As has been previously noted, thedigital-to-analog converter 30 controls the attenuation of variable gainamplifier 16 which correspondingly adjusts the output of on-channelsignal detector 22 and off-channel signal detector 24 creating a closedloop AGC system. The mechanism for controlling the overall AGC loopresponse is determined by adjusting the update rate and the step sizesfor each given state within the automatic gain control state machinecontroller 28. The step size is the voltage difference betweencontiguous updates at the digital-to-analog converter 30 output, withthe minimum step size being one least significant bit (LSB) of thedigital-to-analog converter 30. Higher step sizes are achieved byincreasing the digital-to-analog converter 30 output to be multiples ofthe LSB voltage change. The update rate is the periodicity betweencontiguous output changes of digital-to-analog converter 30. The updaterate is changed by varying the clock divider ratio and/or internaldivider structures contained within the automatic gain control statemachine controller 28. In addition, FIG. 2 illustrates the ability toenable and disable specific states to further refine the overall AGCresponse by including or excluding states based on the operationalprotocol and modulation-type (application) basis. Additionally, FIG. 2illustrates that a given state can incorporate multiple characteristicsas defined by step size and update rate, each characteristicdifferentiated by a specific count limit that would trigger a transitionbetween characteristics once a given count has been exceeded. The countlimit that would trigger transitions within a given state is independentbetween states and may not be included into every state. The advantageof incorporating count limits for a parameter set in a given state isthat it gives the automatic gain control state machine. controller 28the ability to adapt a response for unknown or unanticipated RFenvironments such as encountering extremely strong or weak RF signalsnecessitating significant step size and update rate changes. As anexample, AGC state 2 represents a received signal strength that is lowerthan desired. In other words, the received signal has a strength that isless than threshold TH1 but greater than threshold TH2. Therefore, thedesired AGC action is to increase the gain which means to decrease anyexisting attenuation. Since this state is adjacent the desired state 3,the update rate, rd_1, is made slow. For this state, the AGC step sizeis one LSB whereas the AGC step size for states further away fromdesired state 3 is significantly larger in order to quicker reach thedesired gain state. For the particular modulation protocol, this stateis enabled and enablement is accomplished by making the bit TH1_DISABLEequal to zero. Also, since state 2 is close to desired state 3, there isno adapt initiation hold-off time which will be discussed below indetail in connection with FIG. 8.

Illustrated in FIG. 3 is an automatic gain control (AGC) method 40 foruse with receiver 10. In a step 42, the AGC is initialized to apredetermined value. The initialization is loading state parameters fromthe modulation protocol and parameter registers 29 into the automaticgain control state machine controller 28, setting the digital-to-analogconverter 30 output to reflect the modulation protocol and parameterregisters 29 parameters and providing sufficient time for the variablegain amplifier 16 to reach the corresponding operational condition asset by the control voltage provided by digital-to-analog converter 30.In step 42, the information provided by on-channel signal detector 22and off-channel signal detector 24 is ignored so that the AGC loop isopen. In a step 44, preset conditions are released from fixed controland the loop is closed. This means that the input signals provided byon-channel signal detector 22 and off-channel signal detector 24 areused by the automatic gain control state machine controller 28 to adjustthe output of digital-to-analog converter 30 until a steady state closedloop operation is achieved.

In a step 46, the automatic gain control state machine controller 28 iscontinuously processing the signal indicators in the form of digitalwords from on-channel signal detector 22 and off-channel signal detector24 and adjusting any changes in operational states based on thepredefined thresholds that were load in previously from the modulationprotocol and parameter registers 29. The system is then allowed tosettle. A determination is then made in a step 48 as to whether trackingis enabled. The term “tracking” refers to whether the AGC is allowed toincrease and decrease gain (i.e. bidirectional) or whether it can onlyadjust the variable gain amplifier 16 in one direction. If tracking isnot enabled, a step 62 is then performed wherein a determination is madeas to whether the received signal is greater than threshold A or whetherthreshold A is disabled. If either condition in step 62 is satisfied,then a step 70 is executed to determine the step size and update ratesthat will be utilized. If neither parameter in step 70 is satisfied, astep 72 is executed; however, if both parameters in step 70 aresatisfied, a step 74 is executed. In step 72 the attenuation isincremented by a step size Si_1 at update rate ri_1, and in step 74 theattenuation is incremented by a size of Si_2 at update rate ri_2. Aswill be described below, these step sizes and update rates must becarefully chosen to provide a stable AGC loop dynamic. If however,neither condition in step 62 is satisfied, then it is known that thereceiver 10 is operating at the proper gain level. Step 64 isimplemented wherein all gain parameters are held at the present valuesto maintain the current attenuation value of variable gain amplifier 16.After completion of any of the steps 64, 72 or 74, a return to step 46occurs wherein the monitoring of the input signal levels by on-channelsignal detector 22 and off-channel signal detector 24 continues.

Assume in step 48 that tracking is enabled and step 50 is executed. Adecision is made by the automatic gain control state machine controller28 whether the received signal has a magnitude that is less than thefirst threshold, Th1, or whether threshold Th1 is disabled. If eithercondition in step 50 is satisfied, a step 56 is executed to determinethe step size and update rates that will be utilized in increasing gainof the variable gain amplifier 16. If neither parameter in step 56 issatisfied, a step 58 is executed; however, if both parameters in step 56are satisfied, a step 60 is executed. In step 58 the attenuation isdecremented by a step size Sd_1 at update rate rd_1, and in step 60 theattenuation is decremented by a size of Sd_2 at the update rate of rd_2.After the completion of step 58 or step 60, a return to step 46 occurswherein the monitoring of the input signal levels by on-channel signaldetector 22 and off-channel signal detector 24 continues. It cantherefore be seen that steps 48, 50, 56, 58 and 60 represent a decrementattenuation branch of the methodology and that steps 62, 64, 70, 72 and74 represent an increase attenuation branch of the methodology and whenboth branches are available for use, bidirectional gain adjustments canbe implemented.

Illustrated in FIG. 4 is an expansion of FIG. 3 wherein more than thefour states that were provided in FIG. 3 are defined. For convenience ofexplanation, the same steps are given the same reference numbers. Itshould be noted that the syntax for the step size and update rateassociated with steps 58, 60, 72 and 74 and the signal strengthdetermination steps 58 and 72 differs between FIG. 3 and FIG. 4 sincethere are additional intervening states in FIG. 4. In other words, thestep size and update rate nomenclature used in FIG. 4 is generic whereinthe FIG. 3 illustration represents the resulting embodiment for X equalstwo. Referring to FIG. 4, if either condition associated with step 50 issatisfied, a step 90 is performed. In step 90, if either condition issatisfied, a threshold comparison will be made in subsequent steps (notshown) until such time as a step decision is not satisfied (i.e. until a“No” decision occurs, assuming that step 56 is not encountered). Upon a“No” decision in step 90, a step size and update determination is madein a resultant step 92. If step 90 results in a “Yes” decision,subsequent decision steps are encountered utilizing different thresholdsettings until such time as a “No” decision is encountered. Once a “No”decision is encountered, a resultant step will define the update rateand step size for that threshold. As the amount of attenuation thatvariable gain amplifier 16 provides is adjusted, successive thresholdswill be encountered within the cascaded structure between the decisionsteps 90 and 56 as indicated by the dotted lines of FIG. 4 interveningsteps 90 and 56. Each decision step will result in a change to the stepsize and update rate. Thus, the AGC response is dynamically adjusted tocreate a specific overall response (very fast initial AGC attack timesand slow, over damped update rates once the desired attenuation level isachieved). It should be apparent that the overall AGC response isdetermined by adjusting the step size and update rate parameters for afamily of thresholds. Different AGC responses can be generated byutilizing different families of thresholds having different step sizesand update rates. Each AGC response corresponding to a given family ofthresholds may be tailored to a specific protocol and modulationrequirement.

It should be emphasized that for every decision step between steps 90and 56 and for every decision step between steps 104 and 70 as noted bythe dotted lines in FIG. 4, there is a paired structure where eachdecision step has a companion “No” resultant step (e.g. decision step90, companion resultant step 92) that determines a corresponding AGCstep size and update rate for that threshold decision. Any number ofpaired decisions and resultant steps can be inserted depending upon thedesired resolution of the overall AGC response.

Illustrated in FIGS. 5 and 6 are graphical representations of multiplethresholds defining boundaries wherein step size and update rates arerespectively determined. In FIG. 5, the values of Sd_1 and Sd_2correspond respectively to steps 58 and 60 of FIG. 3 and determinedecrement attenuation step size relative to the detected input signalswing. In FIG. 6, the values of rd_1 and rd_2 correspond respectively tosteps 58 and 60 of FIG. 3 and determine the update rate relative to thedetected input signal swing. It should be further noted that step sizeSd_2 is larger than step size Sd_1 and update rate rd_2 is faster thanupdate rate rd_1 resulting in a fast AGC attack for initial increases ingain. The region defined between Threshold A and Threshold 1 is a steadystate condition determined by step 64 of FIG. 3. Step size Si_1 and Si_2correspond to steps 72 and 74, respectively, of FIG. 3 and determinesincrement attenuation step size relative to the detected input signalswing. In FIG. 6, the values of ri_1 and ri_2 correspond respectively tosteps 72 and 74 of FIG. 3 and determine the update rate relative to thedetected input signal swing when incrementing the attenuation. It shouldbe noted that Si_3 and ri_3 are for illustrative purposes only and wouldrequire additional decision steps to implement as described above inconnection with FIG. 4. Note also that Si_2 and ri_2 are respectivelygreater than Si_1 and ri_1 resulting in a fast attenuation response asthe input signal swing approaches compression. In addition, Si_2 andri_2 utilized in increasing attenuation are significantly larger thanSd_2 and rd_2 utilized in decreasing attenuation. This affordsprotection against compression while simultaneously providing for aslower AGC response that will not track out the AM component of themodulated information. In addition, it should be apparent that theregions defined by the family of thresholds do not have to be equallyspaced as illustrated in FIGS. 5 and 6 but are determined by a designerfor specific modulation and protocols. The illustrated decrement andincrement step sizes of FIG. 5 allow fast initial increment ofattenuation improving attach times for very weak signal conditions whilestill providing a slow overdamped response close to the desiredattenuation level as required by linear modulation.

Illustrated in FIG. 7 is a flowchart of the automatic gain controlmethod illustrated in FIG. 4 where tracking enable step 48 is set to“No”. Generally, this results in a “peak detect” AGC response where theattenuation is set to the maximum detected input signal level. Thismethod is utilized specifically where extremely fast initial AGC attacktimes are required in order to synchronize quickly to received signalswhere the timing of said signals is unknown (e.g. TETRA DMO mode). Oncethe received signal has been acquired, the tracking enable 48 in FIG. 4can be set to “Yes” resulting in a bidirectional AGC response. Thecascaded method illustrated in FIG. 7 and the operation therein issimilar in form and function to previously described methodologies ofFIGS. 3 and 4 for each paired decision step and associated resultantstep between steps 104 and 70. The FIG. 7 flowchart may also representan AGC methodology wherein only a single direction of AGC control isimplemented and no tracking enable step 48 is implemented. In this form,there is no bidirectional response capability.

Illustrated in FIG. 8 is an extension to the AGC methodology describedin FIGS. 3 and 4 following steps 56 and 70. While step 70 (attenuationincrement) is expressly described herein, it should be appreciated thatthe method used in FIG. 8 is equally applicable to step 56 (attenuationdecrement) of FIGS. 3 and 4. Assume that the received signal is greaterthan the X threshold or that the X threshold is disabled. Therefore, astep 142 is performed wherein a counter is incremented to track how manyiterations through the decision step 70 have taken place. The counterindex is compared to a predefined limit known as a preamble in a step144. As long as the number of iterations as indicated by the counterindex is less than the specified limit, a standard step size and updaterate as specified in step 74 is utilized. If however, the counter indexexceeds the specified limit, it is known that the AGC latency is largeand significant changes to the AGC response must be adopted. This isimplemented by performing a step 146 upon a “Yes” condition in step 144,wherein the step size and update rate are significantly increasedthereby maximally accelerating the AGC response.

By now it should be appreciated that there has been provided a receiverthat provides a common platform from which to operate in multipleprotocols wherein different modulation strategies are adopted and evenlyscaled on a real-time basis. The AGC structure taught herein and theassociated methodology can be easily adapted to any present and futuremodulation strategies and protocol implementations. There has beenprovided a generic, flexible method of adapting AGC circuitry tomultiple modulation techniques and protocols while being host processorindependent. The AGC loop control is based on preprogrammed settingssuch as multiple state thresholds, DAC step size, update rate and whichstate, if any, is disabled. By adjusting the update rate and/or stepsizes stored in the modulation protocol and parameter registers 29, thereceive delays may be varied to implement various modulation protocols.The method disclosed herein is directly transferable for use in any ofnumerous hardware implementations. No fundamental change of the gaincontrol loop dynamics is required as signal or protocol requirements arechanged. To implement various protocols, the threshold values arechanged between states as well as the digital-to-analog converter 30step size and update rates. Each threshold value generates a uniqueresponse or signal gain. A plurality of contiguous states or rangeswithin the digital state machine 26 allow AGC control wherein each statehas an independently settable gain update rate and associated gain stepsize. Asymmetric AGC responses can also be achieved by selectivelydisabling or enabling a particular AGC state. The AGC responses that canbe implemented include “ramp up” and hold and “ramp down” and holdresponses. Within a given state of the state machine 26, a preamblesequence may be set to allow response of the state to initiate apredetermined number of iterations at a different update rate ordifferent step size prior to reverting to a predetermined normaloperating parameter.

Although the method and structure taught herein has been disclosed withrespect to certain specific steps and circuitry, it should be readilyapparent that various alternatives may be used. For example, the clockdivision function provided by clock divider 32 may be shared betweenclock divider 32 and a clock divider (not shown) within the automaticgain control state machine controller 28 wherein clock divider 32performs an initial coarse divide to operate some circuitry within statemachine 26 at a higher clock speed than the actual gain update speed.The number of automatic gain control states is a matter of design choiceand is limited by the number of bits of the output of the on-channelsignal detector 22 and the off-channel signal detector 24. For example,if the outputs of the detectors are three-bit outputs, up to eightautomatic gain control states may be implemented. Any type of A/Dconverter and D/A converter may be used including sigma delta converter,resistive ladder converters, capacitive converters, etc. Multiple hostprocessors may interface with the AGC circuitry through one or morerespective serial peripheral interfaces. The values of step incrementsand decrements and update rates are exemplary and are chosen forpurposes of explanation. The connection between the receiver and thehost processor may be a wireless connection. Although the receivedsignal is discussed herein as an RF signal, the method taught herein isapplicable to other frequency ranges.

In the embodiments discussed herein there has been provided an apparatusfor receiving and processing a modulated signal. The apparatus has aninterface for interfacing with a host processor. An automatic gaincontrol state machine is coupled to the interface for receiving controlinformation indicative of a modulation protocol. The automatic gaincontrol state machine is selectably configurable for automatic gaincontrol in accordance with any one of a plurality of modulationprotocols. A storage location is coupled to the interface for receivingand storing the control information and is coupled to provide thecontrol information to the automatic gain control state machine. Atleast one signal detector is coupled to detect and provide an indicationof a signal strength of the modulated signal to the automatic gaincontrol state machine. The automatic gain control state machine isconfigured to operate in each of a plurality of selectable states, eachstate being selected depending on a detected signal strength of themodulated signal. An intermediate frequency generation circuit includesat least one of a mixer and a filter. The at least one signal detectorhas an off-channel signal detector that is coupled to receive a radiofrequency modulated signal on an input side of the intermediatefrequency generation circuit to provide a digital indication of signalstrength of the radio frequency modulated signal to the automatic gaincontrol state machine. An on-channel signal detector receives anintermediate frequency modulated signal on an output side of theintermediate frequency generation circuit and is coupled to provide adigital indication of signal strength of the intermediate frequencymodulated signal to the automatic gain control state machine. A storagelocation is coupled to the interface for receiving and storing thecontrol information and coupled to provide the control information tothe automatic gain control state machine. An attenuation circuit iscoupled to receive at least one attenuation control signal from theautomatic gain control state machine. The automatic gain control statemachine provides the at least one attenuation control signal dependingon the control information stored in the storage location and the signalstrengths of the modulated signals. The attenuation circuit has avariable gain amplifier and perhaps even a step attenuator. An antennais coupled to provide the radio frequency signal to the attenuationcircuit. A variable gain amplifier is coupled to receive an input radiofrequency signal and provides an amplified radio frequency signal. A DACis coupled to receive a digital control signal from the automatic gaincontrol state machine and to provide an analog control signal to thevariable gain amplifier depending on the control information and thesignal strengths of the modulated signals. The interface is a serialperipheral interface and the apparatus further has a host processor, thehost processor being coupled to the serial peripheral interface toprovide the control information indicative of the modulation protocol tobe used by the apparatus for communication with other apparatus usingthe modulation protocol. The approach of the plurality of selectablestates includes at least one of the group of characteristics consistingof automatic gain control action, update rate, step size and an adaptinitiation holdoff time. Each of the plurality of selectable states isdefined by selectable signal strength threshold values. A number of theplurality of selectable states is programmable via the interface. A taskspecific AGC control circuit is coupled to receive at least oneindication of a signal characteristic and coupled to provide an AGCcontrol signal for controlling gain of an AGC loop, wherein the AGCcontrol circuit is configured to control the gain of the AGC loop inaccordance with a plurality of states. Each state corresponds to aselectable range of the signal characteristic and to at least oneprogrammable threshold defining at least one such range. At least onegain control stage is coupled to the task specific AGC control circuit.The at least one gain control stage controls gain of a signal dependingon the AGC control signal. The at least one programmable threshold isselected based upon which one of a plurality of modulation protocols isselected. The at least one signal characteristic includes apeak-to-average signal swing indication and a signal strength indicationof a received signal. An AGC loop is formed by the at least one gaincontrol stage and has at least one of the group consisting of a stepattenuator and a low noise amplifier, a task specific AGC controlcircuit, and a detector stage providing at least one of the indications(e.g., the signal strength indication) of the signal characteristics.The indication is at least one of a D.C. voltage or a digital valueproportional to the received signal. In one form, the receiver has aninterface for interfacing with a host processor; a task specificprocessor for automatic gain control, the task specific processor beingcoupled to receive information from the host processor for determiningautomatic gain control parameters, the task specific processor beingconfigured to operate independently of the host processor. Informationfrom the host processor includes at least one of the group consisting ofsignal range information, signal strength threshold information,automatic gain control update rate information, and automatic gaincontrol step size information. The task specific processor is configuredto operate according to a gain control function which is continuouswithin each of a plurality of signal strength ranges and which isnondifferential at each threshold at an edge of each range. An automaticgain control method includes initializing an automatic gain controlstate machine to a set of preset conditions detecting a signalcharacteristic of a signal to provide a detected signal characteristic,and controlling gain of the signal by a gain stage using the detectedsignal characteristic, wherein the gain is controlled over a pluralityof ranges of the signal characteristic according to a gain controlfunction which is continuous within each of the plurality of ranges andnondifferential at an edge of each of the plurality of ranges. Thesignal characteristic is signal strength. The method further includescomparing the signal strength to a threshold value, the threshold valuedefining an end point of a range of signal strength; controlling gainaccording to a first signal transfer function if the signal is less thanthe threshold value; and controlling gain according to a second signaltransfer function if the signal is greater than the threshold value. Adetermination is made whether the threshold value is enabled prior toany comparing of the signal strength to the threshold value. Thecomparing of the signal strength to the threshold value is performedonly if the threshold value is enabled. The signal characteristic issignal strength. The method further includes comparing the signalstrength to a threshold value, the threshold value defining an end pointof a range of signal strength selecting a first attenuation step size ifthe signal is less than the threshold value, and selecting a secondattenuation step size if the signal is greater than the threshold value.Gain is controlled using at least one of the first attenuation step sizeand the second attenuation step size. The signal strength is to becontrolled towards a programmable target operating range and away from aplurality of operating ranges outside the target operating range, and anattenuation step size for an operating range outside the targetoperating range depends at least in part on the magnitude of thedifference of a threshold of the operating range outside the targetoperating range and a threshold of the target operating range. Adetermination is made to indicate how much time that the automatic gaincontrol state machine has been in a particular state and an adapt stepsize is selected if the time exceeds an adapt holdoff value. Theindication of time is a count of a number of cycles or iterations thatthe automatic gain control state machine has been in the particularstate. A first update rate is selected if the signal is less than thethreshold value, and a second update rate is selected if the signal isgreater than the threshold value. Gain is controlled using one of thefirst attenuation step size or the second attenuation step size and oneof the first update rate or the second update rate. The signalcharacteristic is signal strength. The signal strength is compared to athreshold value, the threshold value defining an end point of a range ofsignal strength. A first update rate is selected if the signal is lessthan the threshold value, and a second update rate is selected if thesignal is greater than the threshold value. Gain is controlled using oneof the first update rate or the second update rate. A determination ismade of an indication of time that the automatic gain control statemachine has been in a particular state. An adapt update rate is selectedif the time exceeds an adapt holdoff value. A determination is madewhether bidirectional gain tracking is enabled and the gain iscontrolled bidirectionally if tracking is enabled. Gain is controlledunidirectionally if tracking is not enabled. A determination is made ifbidirectional gain tracking is enabled and attenuation of the signalstrength is increased if bidirectional tracking is not enabled.Attenuation of signal strength is increased and decreased ifbidirectional tracking is enabled. Initializing the automatic gaincontrol state machine includes loading state parameters from a storagelocation to the automatic gain control state machine and setting adigital-to-analog converter to an output value reflective of the stateparameters by the automatic gain control state machine. Gain of thesignal is controlled by the gain stage under control of thedigital-to-analog converter without influence by signal characteristicsof the signal being gain controlled. The automatic gain control statemachine is released from the set of preset conditions to close anautomatic gain control loop. The state parameters include at least onefrom the group consisting of range edge threshold values, gain actioninformation, step size information, update rate information, and adaptperiod information. In a receiver having an AGC controller, the receiveradapted to interface with a host processor via an interface, a methodincludes controlling an AGC loop within the receiver using an AGC statemachine implemented within the receiver to affect at least one ofattenuation and gain of a signal in a first way if signal strength ofthe signal is in a first programmable range. The AGC loop is controlledusing the AGC state machine to affect at least one of attenuation andgain of the signal in a second way if the signal strength is in a secondprogrammable range.

Benefits, other advantages, and solutions to problems have beendescribed above with regard to specific embodiments. However, thebenefits, advantages, solutions to problems, and any element(s) that maycause any benefit, advantage, or solution to occur or become morepronounced are not to be construed as a critical, required, or essentialfeature or element of any or all the claims. As used herein, the term“couple” is intended to cover direct connections as well as connectionsmade via an intervening coupling element or elements. As used herein,the terms “comprises,” “comprising,” or any other variation thereof, areintended to cover a non-exclusive inclusion, such that a process,method, article, or apparatus that comprises a list of elements does notinclude only those elements but may include other elements not expresslylisted or inherent to such process, method, article, or apparatus.

1. An apparatus for receiving and processing a modulated signal, the apparatus comprising: an interface for interfacing with a host processor; and an automatic gain control state machine coupled to the interface for receiving control information indicative of a modulation protocol, the automatic gain control state machine receiving a signal strength of a signal, comparing the signal strength to a threshold value that defines an end point of a range of signal strength, and controlling gain of the signal according to either a first signal transfer function if the signal is less than the threshold value or a second signal transfer function if the signal is greater than the threshold value, the automatic gain control state machine controlling gain of the signal over a plurality of ranges of the signal characteristic according to a gain control function which is continuous within each of the plurality of ranges and modified to be non-continuous at an edge of each of the plurality of ranges.
 2. The apparatus of claim 1 further comprising: a storage location coupled to the interface for receiving and storing the control information and coupled to provide the control information to the automatic gain control state machine.
 3. The apparatus of claim 1 further comprising: an intermediate frequency generation circuit including at least one of a mixer and a filter; an off-channel signal detector coupled to receive a radio frequency modulated signal on an input side of the intermediate frequency generation circuit to provide a digital indication of signal strength of the radio frequency modulated signal to the automatic gain control state machine; and an on-channel signal detector coupled to receive an intermediate frequency modulated signal on an output side of the intermediate frequency generation circuit and coupled to provide a digital indication of signal strength of the intermediate frequency modulated signal to the automatic gain control state machine.
 4. The apparatus of claim 3 further comprising: a storage location coupled to the interface for receiving and storing the control information and coupled to provide the control information to the automatic gain control state machine; and an attenuation circuit coupled to receive at least one attenuation control signal from the automatic gain control state machine, the automatic gain control state machine providing the at least one attenuation control signal depending on the control information stored in the storage location and the signal strengths of the modulated signals.
 5. The apparatus of claim 4 wherein the attenuation circuit comprises a variable gain amplifier.
 6. The apparatus of claim 5 wherein the attenuation circuit comprises a step attenuator.
 7. The apparatus of claim 4 further comprising an antenna coupled to provide the radio frequency signal to the attenuation circuit.
 8. The apparatus of claim 3 further comprising: a variable gain amplifier coupled to receive an input radio frequency signal and to provide an amplified radio frequency signal; and a DAC coupled to receive a digital control signal from the automatic gain control state machine and to provide an analog control signal to the variable gain amplifier depending on the control information and the signal strengths of the modulated signals.
 9. The apparatus of claim 4 wherein the interface is a serial peripheral interface, the apparatus further comprising the host processor, the host processor being coupled to the serial peripheral interface to provide the control information indicative of the modulation protocol to be used by the apparatus for communication with other apparatus using the modulation protocol.
 10. The apparatus of claim 1 wherein the automatic gain control state machine comprises a plurality of selectable states that includes at least one of the group of characteristics consisting of: automatic gain control action, update rate, step size and an adapt initiation holdoff time.
 11. The apparatus of claim 1 wherein the automatic gain control state machine comprises a plurality of selectable states that are defined by selectable signal strength threshold values.
 12. The apparatus of claim 11 wherein a number of the plurality of selectable states is programmable via the interface.
 13. An automatic gain control method comprising: initializing an automatic gain control state machine to a set of preset conditions; detecting a signal strength of a signal to provide a detected signal characteristic; comparing the signal strength to a threshold value, the threshold value defining an end point of a range of signal strength; and controlling gain of the signal by a gain stage using the detected signal characteristic, wherein the gain is controlled over a plurality of ranges of the signal characteristic according to a gain control function which is continuous within each of the plurality of ranges and nondifferential at an edge of each of the plurality of ranges, the gain stage controlling gain according to a first signal transfer function if the signal is less than the threshold value and according to a second signal transfer function if the signal is greater than the threshold value.
 14. The automatic gain control method of claim 13 further comprising: determining if the threshold value is enabled prior to any comparing of the signal strength to the threshold value; wherein the step of comparing the signal strength to the threshold value is performed only if the threshold value is enabled.
 15. The automatic gain control method of claim 13 wherein the signal characteristic is signal strength, the method further comprising: comparing the signal strength to a threshold value, the threshold value defining an end point of a range of signal strength; selecting a first update rate if the signal is less than the threshold value; and selecting a second update rate if the signal is greater than the threshold value.
 16. The automatic gain control method of claim 15 further comprising controlling gain using one of the first update rate or the second update rate.
 17. The automatic gain control method of claim 15 further comprising: determining an indication of time that the automatic gain control state machine has been in a particular state; and selecting an adapt update rate if the time exceeds an adapt holdoff value.
 18. The automatic gain control method of claim 13 further comprising: determining if bidirectional gain tracking is enabled; controlling the gain bidirectionally if tracking is enabled; and controlling the gain unidirectionally if tracking is not enabled.
 19. The automatic gain control method of claim 13 further comprising: determining if bidirectional gain tracking is enabled; increasing attenuation of the signal strength if bidirectional tracking is not enabled; and increasing and decreasing attenuation of signal strength if bidirectional tracking is enabled.
 20. An automatic gain control method comprising: initializing an automatic gain control state machine to a set of preset conditions; detecting a signal strength of a signal to provide a detected signal characteristic; controlling gain of the signal by using the detected signal characteristic, wherein the gain is controlled over a plurality of ranges of the detected signal characteristic according to a gain control function which is continuous within each of the plurality of ranges; comparing the signal strength to a threshold value, the threshold value defining an end point of a range of signal strength; selecting a first attenuation step size if the signal is less than the threshold value; and selecting a second attenuation step size if the signal is greater than the threshold value.
 21. The automatic gain control method of claim 20 further comprising controlling gain using at least one of the first attenuation step size and the second attenuation step size.
 22. The automatic gain control method of claim 20 wherein the signal strength is to be controlled towards a programmable target operating range and away from a plurality of operating ranges outside the target operating range, and an attenuation step size for an operating range outside the target operating range depends at least in part on the magnitude of the difference of a threshold of the operating range outside the target operating range and a threshold of the target operating range.
 23. The automatic gain control method of claim 20 further comprising: determining an indication of time that the automatic gain control state machine has been in a particular state; selecting an adapt step size if the time exceeds an adapt holdoff value.
 24. The automatic gain control method of claim 20 wherein the indication of time is a count of a number of cycles or iterations that the automatic gain control state machine has been in the particular state.
 25. The automatic gain control method of claim 20 further comprising: selecting a first update rate if the signal is less than the threshold value; and selecting a second update rate if the signal is greater than the threshold value.
 26. The automatic gain control method of claim 25 further comprising controlling gain using one of the first attenuation step size or the second attenuation step size and one of the first update rate or the second update rate.
 27. An automatic gain control method comprising: initializing an automatic gain control state machine to a set of preset conditions; detecting a signal characteristic of a signal to provide a detected signal characteristic; controlling gain of the signal by a gain stage using the detected signal characteristic, wherein the gain is controlled over a plurality of ranges of the signal characteristic according to a gain control function; determining if bidirectional gain tracking is enabled; controlling the gain bidirectionally if tracking is enabled; and controlling the gain unidirectionally if tracking is not enabled.
 28. An automatic gain control method comprising: initializing an automatic gain control state machine to a set of preset conditions; detecting a signal characteristic of a signal to provide a detected signal characteristic; controlling gain of the signal by a gain stage using the detected signal characteristic, wherein the gain is controlled over a plurality of ranges of the signal characteristic according to a gain control function; determining if bidirectional gain tracking is enabled; increasing attenuation of the signal strength if bidirectional tracking is not enabled; and increasing and decreasing attenuation of signal strength if bidirectional tracking is enabled.
 29. The automatic gain control method of claim 28 wherein the gain control function further comprises a gain control function that is continuous within each of the plurality of ranges and which is substantially modified at an edge of the plurality of ranges. 